Why don't earliest sunset(15 Dec) and latest sunrise (31 Dec) occur on the shortest day ?

Answer 1

Hi Richard!

You're further north than I am (I'm in New York) so for me the dates of earliest sunset and latest sunrise respectively are December 8th and January 4th. The further you are from the equator, the closer the dates fall to the solstice, and indeed in the tropics the earliest sunset falls in November! I'll use my dates for New York (41 degrees North), but the principle is the same throughout the north temperate zone.

The winter solstice (December 22 this year) is the shortest day of the year, but it is not the earliest sunset, or the latest sunrise.

Lots of people think that the earliest sunset, the latest sunrise and the shortest day are all on the same day, the winter solstice, which this year falls just after 1 a.m. on December 22 here in New York. In reality, though, each falls on a different day.

For most of the year, the apparent northward and southward motion of the sun governs the changing times of sunrise and sunset. When the sun is moving southward, as it is in September, days in the Northern Hemisphere grow shorter.

The path of the sun across the dome of the sky is known to geometry buffs as a "sine curve." In September, when the sun is moving southward at maximum speed, the days grow shorter by over 3 minutes per day. In December, however, the sun "levels off". The southward motion slows and comes to a stop. For about a week, the sun appears to stand still above the Tropic of Capricorn. The shortening of the day's length slows and stops on December 22.

The north-south motion is not the only one, however. Two others are at work, and together they are known as the "Equation of Time.".

One is based on a simple principle of geometry.

Look at the “sine curve” again. The northward/southward component is only a small part of the sun’s daily motion. Mostly, the sun is moving eastward day by day. In December, the sun is moving almost purely eastward. In September, however, a helping of southward is mixed in with the eastward motion.

Think about that. If the sun covers about the same distance in the sky each day, it’s going to make better eastward time in December, when it’s moving purely eastward, than in September when some of its motion is devoted to moving south too. But to our clocks, each day is exactly the same length, 24 hours, the sun’s average time. This means that, since the sun is moving eastward more in December than September, the earth must spin a little longer to catch up with the sun’s farther-than-average eastward motion. In December, it takes about 20 extra seconds of rotation for the earth to catch up, compared to the time it would take in September.

As the shortening of the days finally damps out in early December, the sunset is no longer getting much earlier each afternoon. But the sun is still both rising and setting about 20 seconds later each day than the day before because of the sun's greater-than-average eastward motion. The point is reached, in about the second week of December in north temperate latitudes, where that 20 seconds a day starts to exceed the fading shortening of the day. When that happens, the sunset slowly reverses and begins to come later, even though the solstice is still almost two weeks away.

In the morning, the opposite is happening. The sunrise is still moving later and later each morning, both because of the shortening day and also the extra 20 seconds needed to catch up with the sun’s maximum eastward motion. The shortening stops in time for the solstice, December 22nd, but because of the extra 20 seconds a day, the sunrise continues to move later, even after the solstice. In fact, it skids all the way into the first week of January until the lengthening of days after the solstice finally begins to drown out the extra 20 seconds a day. That’s why the latest sunrise is in the first week of January in north temperate latitudes.

A similar thing happens in June, because in June the sun is also covering more eastward territory each day than at other times. The effect in December is more pronounced than the one in June, however, due to the earth’s elliptical orbit.

In December and January, the earth is moving faster in its orbit than at any other time of the year. This makes the sun appear to move farther each day than at any other season, about 10 seconds further than average. Much the same as with the 20 seconds-per-day, this delays both the sunrise and sunset in December and January, making the earliest sunset here in New York December 8, and the latest sunrise January 4th, about 27 days apart.

In June, however, the earliest sunrise comes on June 15th, while latest sunset is June 27th, only 12 days apart. This is because in June and July, the earth is moving the slowest in its orbit. The difference between the 27 days in December-January and and the 12 days in June is caused by the contrast between the earth’s speed in December and June.

The effect is less pronounced farther from the equator because the northward-southward difference in day's length is greater as you approach the polar regions. The day's length goes from zero to 24 at the Arctic and Antarctic Circles, but at the equator it never varies from 12.

So you see, although the sun is now rising later each afternoon, the days have not quite stopped getting shorter. That won't happen until tonight, when the solctice happens.

Answer 2

The time by the Sun varies by plus and minus 12 mins through the year because our orbit is an Ellipse. This variation happens faster than the rate of change of inclination of the Earths axis at the Solstices. Notice the average of your two days is very close to the solstice date.

Answer 3

Because of the Equation of Time. Solar days vary in length even though our means of keeping track of them do not--- our clock is based on the "mean," or average solar day, bur the real day varies. As a result, the period of daylight is not centered around the same time of day each day. The difference between local noon on a particular day and "average noon" is called the equation of time. It is due to various causes, such as the ellipticity of the Earth's orbit and the tilt of the ecliptic.

Answer 4

Throw out the Nostradamus prophecies that predicted the end of the world in 1999.

Answer 5

Because the apparent motion of the sun is reversing itself, and there's a little lag.

Answer 6

Because it's about time, not duration.